a) Field of the Invention
The invention is directed to a probe element for coordinate measuring systems which can be used in particular for surface analysis or three-dimensional measurement of very small test pieces or specimens.
b) Description of the Related Art
It is known to carry out mechanical sensing with the aid of a probe ball (Neumann, H. J, Koordinatemne.beta.technik, Bibliothek der Technik, volume 41, Verlag moderne Industrie, Munich 1990). Such probe elements have the advantage that all points on their surface maintain a constant distance from the center and they can also be manufactured with great accuracy. Deviations in shape between probe balls are approximately 0.3 .mu.m. However, it is disadvantageous that the measurement results must be corrected for many measuring tasks, e.g. internal and external measurements, since it is not possible to carry out tactile sensing with the center of the probe ball. The point at which the probe ball contacts the surface of the specimen must be known in order to carry out corrections. Since this point is not known, such measurements always involve some error however slight. The dimensions of the probe balls are too large for measuring very small specimens. Another disadvantage consists in that contacting forces of 0.1 to 1 N at the specimen, probe ball and probe ball holder result in deformations leading to errors which cannot always be compensated.
Further, the lateral resolution capacity of a ball may not be favorable because measurement is effected along a spot or area having a diameter upward of a hundredth of a millimeter, rather than on a measurement point, owing to the curvature of the spherical surface and the flattening caused by the measuring force. Consequently, the measured value represents a mean value obtained along this spot or area.
For the reasons described above, measurement errors of less than 1 .mu.m and measurements on very small specimens, e.g. with inner diameters of less than 1 mm, cannot be realized with probe bails.
Measurement signals are obtained with spherical probe balls in that a resiliently supported plate is tilted due to the tactile sensing movement. This opens an electrical contact which triggers acquisition of the measurement signal. This process enables dynamic measurements to be obtained from the movement, but it is not very accurate. Therefore, more recent spherical probes have a stem connected with piezo-resonators which react to pressure and allow higher switching accuracy.
A further increase in measuring accuracy can be achieved by outfitting the probe head with its own three-dimensional precision adjustments, precision measuring systems and adjustable measurement force devices. In this way the probe ball can sense the surface of the specimen while the probe head remains stationary and the probe ball can be guided for short distances along the surface (scanning).
The disadvantages connected with the probe ball and contacting forces persist in spite of these refinements.
Optical probe elements for coordinate measurement are also known (Gussek, B., Bartel, R., Hoffmann, W., "Ein Mikrotaster erfa.beta.t beruhrungslos Profile im optischen Tastschnitt", Feinwerktechnik und Me.beta.technik 98 (1990) 10, pages 401-405). Probe elements of this type are primarily used with soft specimens. Their advantages consist in that they measure without contact and without exerting force and, in contrast to the ball, the location of the optical contact point is known.
A geometric triangulation method has a resolution of less than 0.1 .mu.m in the vertical direction. However, measurement errors of up to several micrometers can result from the optical characteristics of the surface of the test specimen.
An autofocussing method such as that mentioned above requires surfaces with sufficient reflectivity and also has measurement errors of up to several micrometers depending on reflectivity. The diameter of the light beam or focus is several micrometers and limits the lateral resolution capability of these systems. Conventional methods for tactile sensing of surfaces in coordinate measurement techniques are unsuitable for measurement tasks requiring a lateral resolution capability and measurement errors of less than 1 .mu.or measurement lengths of less than 1 mm for internal measurements. Thus, conventional three-dimensional measurement techniques cannot be used for measuring teeth with modules of less than 0.01 mm, bore holes with diameters of less than 1 mm, threads of less than M1 and test specimens in micromechanics.
Further, arrangements are known in which surface structures are measured by micro-probe needles and piezo-resonators. However, these arrangements are not suitable for purposes of three-dimensional measurement technique, since a three-dimensional measurement head does not appear practical for the measuring elements described in these arrangements. The contact between the oscillating needle fastened to the piezo-resonator and the surface of the test specimen is determined by measuring either the change in the resonator parameters or the contact forces (DE-OS 4035076.2, DE-OS 4035084.2).